1. Field of the Invention
The invention relates to an automatic gain control (AGC) and digitizing circuit particularly suitable for use in digital video image processing and/or display systems.
2. Description of the Prior Art
A video camera is used to produce an electrical representation of an image for many uses, one of which is to provide a video image on a remote monitor in security applications. In some of these applications, a camera is placed at a particular location and scans a particular scene frequently over an extended period of time. Unfortunately, background light which forms part of this scene frequently changes over this period. In order to produce a video image on the remote monitor having proper brightness and contrast, the video signal produced by the camera must be constantly adjusted whenever the background light changes. This requires that the gain and DC level of the video signal change in response to changes in the intensity of the background light; otherwise, the remotely displayed image will either be too light or too dark and hence difficult to perceive.
A video signal is comprised of negative going horizontal and vertical synchronization pulses (hereinafter collectively referred to as the "synch" pulses) having a maximum peak amplitude of -0.4 volts which sandwich analog video (picture) information that varies between +1.0 volt, for a pure white signal, to 0.0 volts or ground, for a true black signal.
One well-known method to ensure that a video signal is maintained at an appropriate level is to connect an automatic gain control (AGC) circuit to the output of the camera. This circuit, fed by the video signal produced by the camera, responds to any changes in the video signal caused by changes in the intensity of the light comprising the scene and then adjusts the video signal accordingly.
Furthermore, in video applications which use digital processing techniques, the video signal produced by a camera is digitized by a suitable analog-to-digital (A/D) converter prior to subsequent image processing and/or display. In these applications, an AGC is essential in order to ensure that the picture information consumes the entire dynamic range of the A/D converter.
Several different video AGC circuits appear in the art. However, all of these circuits disadvantageously possess various drawbacks. For example, one such circuit which sees very common use in the art is the so-called "DC restorer". In theory, this circuit operates under the assumption that the gain of the video signal is correct. This circuit then determines the voltage corresponding to the blackest area in the video image and adjusts the video level to ensure that this voltage level lies within a proper range. However, in practice, this type of circuit does not operate according to this theory for the following reason. DC restorer circuits look for the most negative voltage, and adjust the DC level of the video signal based on this voltage under the assumption that this voltage corresponds to the blackest area in the image when in actuality the most negative voltage actually corresponds to the negative peaks of the synch pulses. Since DC restorer type AGC circuits do not discriminate between true black signals on the one hand and the negative peaks of synch pulses on the other hand, these circuits unnecessarily compress the range of the video signal by upwards of 30-40%. Use of DC restorer circuits is particularly disadvantageous whenever the video signal is fed to an analog-to-digital (A/D) converter. Here, 30-40% of the dynamic range of the converter is needlessly lost by shifting the level of the entire video signal including the synch pulses--which carry no picture information--so that the entire signal lies within the useable range of the converter.
Another type of prior art AGC circuit feeds the video signal produced by a video camera through an analog multiplier, typically a transconductance amplifier. Here, the AGC includes a detector which determines the brightest and darkest areas in the image, and adjusts the gain of the AGC, by appropriately varying the magnitude of one of the input voltages applied to the multiplier, so that the voltages corresponding to these areas never exceed a pre-selected range. Unfortunately, this type of AGC circuit is not only extremely complex and expensive, but also exhibits non-linear performance inasmuch as analog multipliers--particularly transconductance amplifiers--possess some non-linearity. Moreover, these AGC circuits require a viewer to properly adjust brightness and contrast controls in order to produce peak performance. Since few viewers know how to properly adjust these controls, such circuits often seem to produce inferior results.
In addition, video AGC circuits known to the art, which are employed in digital image processing systems, utilize the input analog voltage applied to the A/D converter as a feedback voltage to determine the proper amount of AGC gain and level change. As a result, these AGC circuits can not eliminate the small amounts of DC offset voltages that the A/D converter typically injects into its output signal.
Moreover, the background light which comprises only a portion of a scene will often change. In these situations, dependent upon where that portion is situated relative to the rest of the scene, it may not be desireable to alter the level of the video signal produced by the camera. For example, if that portion is in an area of no interest to a viewer but nonetheless becomes very bright and if the video signal is adjusted in response to this change in intensity, then the remainder of the image will become too dark and lose sufficient contrast to permit adequate viewing. Hence, any activity occurring in the area of interest will be not be readily detectable on a remote monitor. In an attempt to ameliorate this problem, some prior art AGC circuits average any changes in the light intensity over the entire scene in order to produce an average value and then adjust the video signal in response to this average value rather than in response to any isolated peak changes in the light intensity. While this well-known technique provides satisfactory results in most situations, difficulties arise where a white or light colored object is displayed against a black background. Here, after a video signal has been modified by the AGC, truly black areas tend to become too dark (i.e. too low in voltage) and are not displayed at all, and areas that are not quite as black are displayed as being black. This, in turn, causes a phenomena known as "tilt" to appear in the image wherein one portion of a supposedly black area gets darker relative to other portions of the same black area.
Thus, a need has existed in the art for an inexpensive and simple video AGC circuit, particularly suited for use in conjunction with a A/D converter. This circuit should: first, continuously modify the picture information, including its DC level, as well as change the gain of the converter, if necessary, to ensure that the picture information consumes the entire dynamic range of the converter; second, respond to light intensity changes occurring in any selected area of the scene and ignore changes occurring in other areas; and third, eliminate any offsets injected by the A/D converter.